1. Field of the Invention
The present invention relates to lubricating oils and to a method for improving the oxidative stability of such oils by using a combination of additives.
2. Description of the Related Art
The literature contains numerous examples of combinations of compounds to give improved oxidation stability to lubricating oils.
Amine antioxidants have been found to act in concert with phenolic antioxidants to give improved oxidation stability. This form of cooperative interaction is termed homosynergism because both compounds act by the same stabilization mechanism, in this case a free radical decomposing mechanism. See Mescina and Karpukhina, Zik. Maizus. Neflekhimiya 12, 731 (1972).
Compounds which act by different stabilizing mechanisms can give rise to heterosynergism. For example, alkylated diphenylamine, a radical scavenger in combination with organosulfur compounds, a hydroperoxide decomposer, have been shown to lead to increased oxidation stability in the ASTM D943 oxidation test, see Rosberger, M. Chemistry and Technology of Lubricants, page 108, VHC Publishers Inc., New York, 1992. The ASTM D943 test measures the time required for a test oil to attain a Total Acid Number (TAN) of 2.0 mg KOH/g. Another typical oxidation test used as an industry standard is the Rotary Bomb Oxidation Test (ASTM D 2272) in which oxidation life is measured in minutes prior to an oxygen pressure drop of 25 pounds.
U.S. Pat. No. 3,346,496 is directed to lubricating oils containing carbodiimides as antioxidants. The patent states that the use of carboidiimides makes it possible to substantially improve the resistance of various types of lubricants to oxidative attack. The lubricants can be based on mineral oils or synthetic oil base stocks such as polyethers or polyether esters. The carbodiimides are also reported as being effective protective agents against corrosion and as being capable of keeping decomposition products formed during the lubricating process in solution. The patent states that the carbodiimides can be added to the lubricants in combination with diphenyl amine anti oxidants or hydroquinolines and that, surprisingly, a synergistic effect is achieved. Review of the data presented in U.S. Pat. No. 3,346,496 and the different pathways by which carbodiimides and diphenyl amines act, however, show that these statements are not correct.
The carbodiimides react with acidic molecules (carboxylic acids, inorganic acids . . .) to give neutral products. Thus, if an oil has acidic components, adding a carbodiimide will lower the Total Acid Number (TAN) of the oil. The oxidation of mineral oils is widely understood to take place through a free radical mechanism. Some of the reaction products of this oxidation process are organic acids such as carboxylic acids. The presence of these acids, however, does not significantly promote the oxidation of the mineral oil. Another way of saying this is that the free radical oxidation of mineral oils is not acid catalyzed. Molecules that acts as antioxidants for mineral oils do so by either interrupting the free radical propagation mechanism of the oxidation process or by decomposing free radical initiators such as hydroperoxides. By doing this, they slow down the oxidative degradation of the oils.
The TAN of an oil is often used as an indication of the extent to which the oil has oxidized. Again, this is because the concentration of acidic molecules in an oil increases as the oil oxidizes and is thus an indirect measure of the extent of oxidation of the oil. The examples shown in U.S. Pat. No. 3,346,496 use the D 943 oxidation test to measure the oxidation life of the oils. This test measures the TAN of the oil. The time it takes for the TAN of the oil to reach 2.0 mg KOH/mg is deemed the oxidation life of the oil for this test. A unique situation is created when an acid scavenging molecule, such as a carbodiimide, is added to an oil. The TAN can no longer be used as a measure of the oxidation life of the oil. The oil will undergo its normal oxidation process but the acidic byproducts of oxidation are effectively removed from the oil and therefore the concentration of acid in the sample does not accurately reflect the extent of oil oxidation.
It is expected that, in a mineral oil which contains both a diphenyl-amine antioxidant and a carbodiimide acid scavenger, the oxidation life of the oil, as measure by the D 943 test, would be approximately equal to the sum of the oxidation life of the same oil with the same concentration of diphenylamine and the same mineral oil with the same concentration of carbodiimide minus the oxidation life of the mineral oil itself (so you do not count it twice). This is because the diphenylamine antioxidant would react to interfere with the oxidation process of the oil until the diphenylamine was depleted. At this point the oil would start to oxidize and produce acidic products. Once formed, these acidic products would react with the carbodiimide. The TAN of the oil would remain low until the carbodiimide was depleted. These two processes are separate events which, for the most part, would happen sequentially.
Table 1 of U.S. Pat. No. 3,346,496 lists TAN data, from D 943 testing, relevant to their invention. Review of the data of U.S. Pat. No. 3,346,496 reveals that the TAN of a naphthene-based oil, with 1% of 2,6,2',6'-tetra-isopropyl-diphenyl-carbodiimide, reaches 2.0 mg KOH/mg after about 510 hours on test. The TAN of the same naphthene-based oil, with 0.2% 4,4'-dimethylbenzyldiphenylamine, would reach 2.0 mg KOH/mg after about 350 hours on test. From Table 1 of U.S. Pat. No. 3,346,496 it can be estimated that the naphthene-bases oil per se reached a TAN=2.0 mg KOH/mg after about 30 hours. Therefore, a formulation in the same naphthene-based oil containing 1% of 2,6,2',6'-tetra-isopropyl-diphenyl-carbodiimide and 0.2% 4,4'-dimethylbenzyldiphenylamine would be expected to reach a TAN of 2.0 mg KOH/mg after about 830 hours on test. The data shows this exact combination to reach a TAN of 2.0 mg KOH/mg after about 915 hours on test. This gives a difference of about 85 hours between the expected lifetime and the measured lifetime. The precision statement for the D 943 test states that the repeatability of the test method is 0.192.times.(mean measurement value). Therefore, the measured value of 915 hours has an error of +/-176 hours. Consequently, the measured value of 915 hours is not statistically different from the expected value of about 830 hours. A synergy has only occurred when the combined effect of two or more agents is greater than the sum of the effects of each of the agents separately. Contrary to the claim, the data presented in the U.S. Pat. No. 3,346,496 shows that the combination of carbodiiminde and diphenylamine are not synergistic.